In a typical cellular radio system, radio or wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UEs) within range of the base stations.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. The first release for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification has issued, and as with most specifications, the standard will likely evolve. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology where the radio base station nodes are connected to a core network (via Access Gateways (AGWs)) rather than to radio network controller (RNC) nodes. In general, the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has what is sometimes termed a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
A key feature of long term evolution advanced (LTE-A) is achieving higher data rates by allowing a user equipment to receive and transmit data on multiple LTE component carriers simultaneously in both uplink and downlink directions, which is referred to as “carrier aggregation.”
In legacy LTE release-8, the UE receives and transmits using a single carrier frequency. To limit UE and base station complexity, the radio and performance requirements are specified for a limited number of channel bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. The legacy LTE release-8 UE supports transmission and reception up to 20 MHz of channel bandwidth. In LTE, orthogonal frequency division multiple access (OFDMA) and single carrier—frequency division multiple access (SC-FDMA) are used as access technologies in the downlink and uplink, respectively. Therefore, the physical radio communication resources are expressed in terms of “resource blocks,” both in the downlink and the uplink. A resource block is a time-frequency resource comprising of one time slot (0.5 ms) in time and 180 KHz or 12 sub-carriers (1 sub-carrier carrier spacing=15 KHz) in frequency. Each legacy LTE release-8 channel bandwidth contains certain number of resource blocks. For instance, a 20 MHz carrier frequency can theoretically accommodate 110 resource blocks. But in order to meet various practical radio requirements, (e.g., modulation quality, spectrum emission mask, etc.), a guard band is required on each flank of the carrier frequency. For example, in order to fulfill at least one example set of radio requirements, only 100 resource blocks are transmitted in both the uplink and the downlink for a 20 MHz carrier frequency.
Carrier aggregation is described at a general level in the feasibility study for 3GPP LTE release-10, i.e., LTE-Advanced (LTE-A) at 3GPP TR 36.815 V9.1.0 (2010-06) available at the 3GPP website www.3gpp.org. Carrier aggregation means that two or more “component carriers” are aggregated in order to support wider bandwidths. Each carrier frequency is referred to as a component carrier. A goal is to aggregate a different number of component carriers of possibly different bandwidths in the UL and the DL. FIG. 1A illustrates an example of aggregated bandwidth of 90 MHz made up of four 20 MHz component carriers and one 10 MHz component carrier, which are all contiguous. FIG. 1B illustrates an example of aggregated bandwidth of 20 MHz made up of four 5 MHz component carriers, two of which are not contiguous and two of which are contiguous.
Carrier aggregation allows a UE to simultaneously receive and/or transmit data over more than one carrier frequency thereby enabling a significant increase in data reception and/or transmission rates. For instance, 2×20 MHz aggregated carriers theoretically provide a two-fold increase in data rate as compared to the data rate for a single 20 MHz carrier. As shown in the examples in FIGS. 1A and 1B, a component carrier may be contiguous or non-contiguous. Non-contiguous carriers may belong to the same frequency band or to different frequency bands. Aggregated bandwidths of up to 100 MHz using up to 5 component carriers are being considered. A hybrid carrier aggregation scheme with contiguous and non-contiguous component carriers is also contemplated for LTE-advanced.
Implementation of the above contiguous carrier aggregation scenarios requires that a network operator own a very large contiguous frequency band, e.g., 80-100 MHz in the same band. But presently, large contiguous frequency allocations, e.g., on the order of 80-100 MHz, either do not exist or may be difficult to acquire in the future. One way to allow an operator to efficiently use larger chunk of available spectrum is to employ an extension carrier or segment which is generally a smaller carrier compared to other component carriers and can be used by an operator to “fill in” the available spectrum.
To illustrate this point, consider an example in which contiguous 80 MHz spectrum is available. An operator can use 4×20 MHz carriers, each with 100 resource blocks (RBs). This configuration maintains backward compatibility with the legacy LTE-release 8 20 MHz carrier in terms of resource blocks (there are 100 resource blocks per 20 MHz in legacy LTE-release 8). A small portion of spectrum of about 6 MHz (which is less than the 20 MHz legacy spectrum) remains unused as now explained. The 20 MHz carrier has a channel bandwidth of 20 MHz, but a transmission bandwidth of 18 MHz for the 100 RBs because there is 1+1 MHz of guard band inside the channel bandwidth, namely, 1 MHz at each end of the 18 MHz. This means that a densely-packed system for LTE-A of 4×20 MHz uses 4×18 MHz=72 MHz out of the 80 MHz to actually carry resource blocks. In other words, the 4×20 MHz carriers use up all of the available spectrum if the four carriers are configured as: (1+18+1)+(1+18+1)+(1+18+1)+(1+18+1). But if instead the four carriers were configured as: (1+18+18+18+18+1) without the 6 intervening guard bands, that provides room for 6 MHz for additional extension carriers. If a 5 MHz extension carrier is added onto 4×20 MHz carriers, then the available 80 MHz can be more efficiently used. Specifically, if the 25 RBs=4.5 MHz are packed next to the 4×18 MHz=4×100RB=4×100*0.18 MHz, then the result is 1+4×18+4.5+1=78.5 MHz. So even though a guard band of small unused spectrum at the edges of the aggregated carriers must be maintained to meet practical radio requirements, which means 100% spectrum utilization is not practically possible, introduction of an extension carrier can significantly improve the spectral efficiency.
One approach for implementing carrier aggregation in LTE-advanced (LTE-A) is to make the extension carrier backward-compatible to legacy LTE-8 carriers, which means all control channels normally found in the LTE-8 carrier are also found in the extension carrier. An alternate approach that is not backward-compatible to LTE-8 carriers uses an extension carrier with only data resource blocks and no control channels. But this static design choice deprives the operator of flexibility to selectively choose an approach that best suits a current network need or situation. In other words, when configuring new spectrum for LTE-A carrier aggregation, the operator either selects a component carrier design that is an LTE backwards compatible component carrier but which suffers a user data inefficiency due to signaling channel overhead associated with that backwards compatibility, or an LTE-10 extension carrier, which is efficient because there is no signaling overhead, but which is not backwards compatible to earlier LTE releases.
What is needed is flexibility to be able to selectively and dynamically implement either approach including one with one or more extension carriers with control channels and another where extension carriers are configured without control channels. Another need is a signaling mechanism to support such flexible carrier aggregation.